* Trial in absentia to further strain once-close ties
* Israel dismisses proceedings as a "show trial"
* Protesters chant anti-Israel slogans outside courtroom
* Indictment seeks multiple life sentences for accused
By Ece Toksabay
ISTANBUL, Nov 6 (Reuters) - Hundreds of protesters chanting "Murderer Israel!" gathered outside an Istanbul court on Tuesday at the start of a trial of a group of former Israeli military commanders charged over the 2010 killing of nine Turks aboard a Gaza-bound aid ship.
The trial in absentia of four of Israel's most senior retired commanders, including the head of the army, has been dismissed by Israel as a politically motivated "show trial" and threatens to further strain already fraught relations.
Ties between the Jewish state and what was once its only Muslim ally crumbled after Israeli marines stormed the Mavi Marmara aid ship in May 2010 to enforce a naval blockade of the Palestinian-run Gaza Strip.
Nine Turks were killed in clashes with activists on board.
The ensuing rift remains raw despite U.S. efforts to encourage a rapprochement between the two regional powers whose alliance was a mainstay of Washington's influence in an unstable region.
Israel and NATO member Turkey, which both border Syria, once shared intelligence information and conducted joint military exercises, cooperation which has since been cancelled.
Several hundred people, many wearing the Arab keffiyeh headscarf around their necks adorned with the Turkish and Palestinian flags, crowded outside the courthouse as witnesses and relatives of those killed in the raid began to arrive.
"Murderer Israel, get out of Palestine!" the crowd chanted as others held up a banner with the words: "What is the difference? Hitler = Israel."
On a board erected outside the courthouse by IHH, the Islamic humanitarian agency that owns the Mavi Marmara, protesters scribbled the slogans: "Israel, your end is near", "Down with Israel", "The revenge of our martyrs will be bitter".
"We want nothing more than for those who are responsible to be punished. We want them to be brought to account for the violation of Palestinian people's rights," said Ummugulsum Yazici, one of the protesters.
Inside the courtroom, the presiding judge began hearing testimony from those who were aboard the flotilla during the 2010 raid. A total of 490 people, including activists and journalists, are expected to give evidence.
Ann Wright, a former colonel in the U.S. army, who was on one of the smaller ships, described how Israeli troops had boarded their vessel, firing paint bullets and tossing stun grenades.
"After serving 29 years in the U.S. army, I retired in 2003 in opposition to the Iraq war," she told the court.
"One of the reasons I went on the flotilla is that I felt compelled to challenge Israeli policy to impose an illegal blockade on Palestine and the U.S. policy to support Israel's illegal actions," she said.
The indictment prepared by a state prosecutor is seeking multiple life sentences for the now retired Israeli officers over their involvement in the nine killings and in the wounding of more than 50 others.
The 144-page indictment names Israel's former Chief-of-Staff Gabi Ashkenazi, and three other senior commanders. It lists "inciting murder through cruelty or torture" and "inciting injury with firearms" among the charges.
Israel has dismissed the case as a "show trial" and "political theatre", saying the accused had not even been notified of the charges.
"This is not a trial, this is a show trial with a kangaroo court. This is a trial taken right out of a Kafka novel, a grotesque political show that has nothing to do with law and justice," Yigal Palmor, an Israeli Foreign Ministry spokesman, told Reuters TV.
Turkey expelled Israel's ambassador and froze military cooperation after a U.N. report into the 2010 incident released in September last year largely exonerated the Jewish state.
That report was meant to encourage a rapprochement between the two countries but ultimately deepened the rift when it concluded that Israel had used unreasonable force but that its blockade on Gaza was legal.
Israeli Prime Minister Benjamin Netanyahu said in July that Israel and Turkey needed to repair their relationship, but attempts to rekindle the strategic relationship have failed.
Turkey has demanded a formal apology, compensation for victims and the families of the dead, and for the Gaza blockade to be lifted.
Israel has voiced "regret", falling short of the full apology demanded, and has offered to pay into what it called a "humanitarian fund" through which casualties and relatives could be compensated. (Additional reporting by Jerusalem Newsroom; Writing by Jonathon Burch; Editing by Andrew Osborn)